The present invention relates generally to the use of ultrasound to observe pulsatile activity in living tissue.
Ultrasound imaging provides a non-invasive means to view internal portions of a patient""s body, and so has proven a useful medical diagnostic tool. Ultrasound waves, typically of a frequency in the range of 2 to 20 MHz, are emitted into the body of the patient and reflected ultrasound waves from the body tissues are received and converted into signals which are processed to produce an image. These images, while usefull, have been limited in that they were two-dimensional and of low resolution. Recently, higher resolution and three-dimensional ultrasound images have become available, but without real-time imaging, as they require many measurements and significant data processing time to produce the images. Even advanced existing ultrasound imaging systems do not allow observation of pulsatile behavior, which can be a valuable diagnostic tool, in the tissue being examined.
A further limitation of existing ultrasound imaging systems is that the ultrasound waves they use undergo significant attenuation when passing through bone, which limits their usefulness for observing certain regions of the body, especially the brain.
U.S. Pat. No. 5,840,018, granted to the present inventor, discloses a method to perform realtime observations on blood vessels in the brain; the contents of this patent are incorporated herein by reference. It includes means and method for processing and analyzing time varying signals associated with pulsatile activity in blood vessels, especially by employing gating circuitry.
U.S. Pat. No. 4,549,533 discloses apparatus and method to generate and direct ultrasound energy over a predetermined region of the body, but the application disclosed therein is only for directing the ultrasound energy to the target region, without detecting reflected ultrasound energy to observe the target region.
U.S. Pat. No. 5,540,230 discloses apparatus and method employing ultrasound to determine the velocity of fluid flowing through a lumen. U.S. Pat. No. 5,394,750 discloses an ultrasound transceiver which includes filtering on received signals to reduce noise in detected signals and which may be employed to determine the velocity profile of blood flow in tissue. Both inventions disclosed therein only allow observation of a single point with no provision for multiple observations in real time.
U.S. Pat. No. 5,787,889 discloses a method and an ultrasound system for three-dimensional ultrasound imaging in real time, but is restricted to a physical image of the tissue being scanned, without observing pulsatility details. It is further limited by operator movement of the ultrasound transducer/receiver over the surface of the patient; indeed, the real-time imaging disclosed therein corresponds to tracking this manual probe movement. Further, ultrasound waves in the frequency range cited therein are subject to significant attenuation when passing through bone, resulting in the above mentioned limitation of applicability.
An aim of the present invention is to provide a method to image a volume of tissue in a subject with ultrasound waves that overcomes the limitations of the prior art by providing a dynamic three-dimensional representation and at a high resolution of pulsatile activity in the selected volume of tissue.
There is thus provided, in accordance with a preferred embodiment of the invention, a method for observing three-dimensional pulsatile activity in a preselected volume of tissue in a subject that includes the following steps:
placing an array of ultrasound probes in association with the surface of the subject, thereby selecting a discrete volume of tissue in the subject;
selecting a generally linear subset of the array, thereby defining a thin slice of the selected volume of tissue;
activating and focusing a selected contiguous portion, having a predetermined curvature, of the selected subset of the array of probes, so that each probe is operative:
to emit ultrasound waves in a preselected frequency waveband and at a preselected range of output intensities, typically with a bandwidth of substantially 0.4 MHz in the frequency range 0.4-40.0 MHz and an output intensity in the range 100-300 mW/cm2, but especially in a waveband selected so as not to be substantially attenuated by bone,
to receive reflected ultrasound energy from the tissue in the preselected frequency waveband, and
to convert the received reflected ultrasound energy into output signals corresponding thereto;
and so that the focusing serves to select a portion of tissue along a line contained within the selected tissue slice which further intersects the linear subset of probes, the volume of the selected portion of the tissue slice defines the pixel size for the image that is desired to be produced, which is in the range 0.1 to 1.0 mm3;
receiving, via the probes constituting the selected contiguous portion of the subarray of probes, the reflected ultrasound energy from the tissue and converting it into output signals corresponding to the reflected ultrasound energy;
processing these output signals from the probes so as to determine pulsatile activity in the selected portion of the selected tissue slice;
providing output data corresponding to the pulsatile activity in the selected portion of the selected tissue slice;
varying the focus of the ultrasound energy along the line within the volume slice and repeating above steps thereby scanning the selected line within the selected tissue slice and providing output data corresponding to the pulsatile activity in successive portions of tissue along the line;
selecting a sequence of contiguous portions of the selected subset of the array and repeating above steps thereby selecting a sequence of lines within the selected tissue slice, thereby scanning the selected tissue slice and providing output data corresponding to the pulsatile activity in successive portions of tissue along the lines within the slice;
selecting a sequence of subsets of the array of ultrasound probes and repeating above steps thereby selecting a sequence of slices within the selected tissue volume, thereby scanning the selected volume and providing output data corresponding to the pulsatile activity in successive slices of tissue within the volume; and
performing tomographic analysis of the plurality of output data, thereby obtaining a three-dimensional image of the pulsatile activity in the selected volume of tissue.
Further in accordance with a preferred embodiment of the present invention, the sequence of slices within the selected tissue volume is a sequence of substantially parallel first slices, and the method further includes the following steps:
selecting additional linear subsets of the array of ultrasonic probes arranged in association with a second tissue slice having a non-parallel alignment with respect to the first tissue slices;
repeating the step of selecting a sequence of subsets of the array arranged in association with additional tissue slices parallel to the second tissue slice; and
performing tomographic analysis of the plurality of output data, thereby obtaining a directional three-dimensional image of the pulsatile activity in the selected volume of tissue.
Before performing the tomographic analysis, additional linear subsets in association with subsequent tissue slices having non-parallel alignment with respect to both first and second tissue slices may further be selected. This, together with repetition of the step of selecting a sequence of subsets, provides more directional information to the three dimensional image of the pulsatile activity in the selected volume of tissue so obtained.
In accordance with the alternative preferred embodiments of the present invention, the step of activating and focusing may include changing the curvature of the selected contiguous portion of the linear subarray of ultrasound probes or adjusting the timing of the activation of the selected contiguous portion of the subarray of ultrasound probes, so as to focus the ultrasound energy variably within the selected tissue slice and to observe pulsatile activity therein. The frequency waveband of the ultrasound waves may also be varied.
Further in accordance with a preferred embodiment of the invention, the step of processing the output signals includes the substeps of:
converting the output signals into a summed output signal associated with the selected portion of the selected tissue slice;
measuring variation in the summed output signal as a function of time; and
observing selected features of pulsatile activity in the time variation of the summed output signal associated with the selected portion of the selected tissue slice, typically by applying gating circuitry to the time variation.
Additionally in accordance with a preferred embodiment of the invention, the substep of observing selected features of pulsatile activity includes the substeps of:
performing spectral analysis of the summed output signal associated with the selected portion of the selected tissue slice to produce a frequency spectrum associated therewith;
selecting a reference pulsatile signal associated with the heart rate of the subject, such as an electrocardiogram signal;
performing spectral analysis of the reference pulsatile signal associated with the heart rate of the subject to produce a frequency spectrum associated therewith; and
comparing the frequency spectrum of the summed output signal associated with the selected portion of the selected tissue slice with the frequency spectrum of the reference pulsatile signal.